02 May 2013

Combination Reflection Nebula and Emission Nebula in NGC 6559 Imaged

The Danish 1.54-metre telescope located at ESO’s La Silla Observatory in Chile has captured a striking image of NGC 6559, an object that showcases the anarchy that reigns when stars form inside an interstellar cloud.

An interstellar cloud of dust, hydrogen, helium and other ionized gases is called a nebula. This region of space is where stars are formed. The materials needed to form stars such as hydrogen and other gases are abundant in a nebula. Stars are formed when dust and gas start to clump together and gain mass. As the mass gets more dense and generates heat, thermonuclear fusion starts and a star starts to form.

NGC 6559 is a nebula that was imaged by the Danish Faint Object Spectrograph and Camera (DFOSC) in La Silla Observatory. The colors radiated of the object are from the light given off by the newly formed stars and because of the gasses present in the center gives the nebula a reddish glow and the dust cloud beside it blocks light which gives the nebula a dark and bluish tinge in that area. It is this combination of gas and dust that makes NGC 6559 both an emission nebula in the red part, and a diffusion nebula in the darker area.Glowing Nebula NGC 6559

NGC 6559 is a cloud of gas and dust located at a distance of about 5000 light-years from Earth, in the constellation of Sagittarius (The Archer). The glowing region is a relatively small object, just a few light-years across, in contrast to the one hundred light-years and more spanned by its famous neighbour, the Lagoon Nebula. Although it is usually overlooked in favor of its distinguished companion, NGC 6559 has the leading role in this new picture.

The gas in the clouds of NGC 6559, mainly hydrogen, is the raw material for star formation. When a region inside this nebula gathers enough matter, it starts to collapse under its own gravity. The centre of the cloud grows ever denser and hotter, until thermonuclear fusion begins and a star is born. The hydrogen atoms combine to form helium atoms, releasing energy that makes the star shine.

These brilliant hot young stars born out of the cloud energise the hydrogen gas still present around them in the nebula. The gas then re-emits this energy, producing the glowing threadlike red cloud seen near the centre of the image. This object is known as an emission nebula.

Video: Reflection and Emission Nebula NGC 6559

But NGC 6559 is not just made out of hydrogen gas. It also contains solid particles of dust, made of heavier elements, such as carbon, iron or silicon. The bluish patch next to the red emission nebula shows the light from the recently formed stars being scattered — reflected in many different directions — by the microscopic particles in the nebula. Known to astronomers as a reflection nebula, this type of object usually appears blue because the scattering is more efficient for these shorter wavelengths of light.

In regions where it is very dense, the dust completely blocks the light behind it, as is the case for the dark isolated patches and sinuous lanes to the bottom left-hand side and right-hand side of the image. To look through the clouds at what lies behind, astronomers would need to observe the nebula using longer wavelengths that would not be absorbed.

The Milky Way fills the background of the image with countless yellowish older stars. Some of them appear fainter and redder because of the dust in NGC 6559.

This eye-catching image of star formation was captured by the Danish Faint Object Spectrograph and Camera (DFOSC) on the 1.54-metre Danish Telescope at La Silla in Chile. This national telescope has been in use at La Silla since 1979 and was recently refurbished to turn it into a remote-controlled state-of-the-art telescope.